(a) Field of the Invention
This invention relates to an engine exhaust particulate after-treatment system.
(b) Description of the Related Art
For the purpose of purification of exhaust gases from diesel engines, there has been recently employed a technology for trapping exhaust particulates such as carbon contained in exhaust gases by a particulate filter disposed in an exhaust path of the engine to prevent atmospheric release.
In such a diesel engine provided with a particulate filter, when the amount of exhaust particulate accumulated on the particulate filter reaches a saturation amount which is the limit to which the particulate filter can accumulate exhaust particulates, it is necessary to burn the accumulated exhaust particulates to regenerate the function of the filter.
As an example of filter regeneration techniques, there is known one for burning exhaust particulates in such a manner to raise the exhaust gas temperature by operating a heater provided in the particulate filter or by retarding the timing of fuel injection from a fuel injection valve behind that during normal operation to promote after-burning.
In the above known technique, however, if the engine operating condition changes to a deceleration condition in the course of regeneration of the particulate filter, this may cause a problem that the temperature of the particulate filter may rise suddenly to deteriorate the durability thereof.
More specifically, while the particulate filter during regeneration raises its temperature as a result of the burning of exhaust particulates, it suddenly raises its temperature, when the exhaust gas flow rate decreases due to vehicle deceleration, because the particulate filter attenuates its cooling action caused by the passage of exhaust gases therethrough (a cooling action caused by heat exchange between the exhaust gases and the particulate filter).
Japanese Examined Patent Publication No. 5-11205 discloses a technique for suppressing dissolution loss of a particulate filter, when the engine operating mode shifts from a high load to idling condition and the temperature of the particulate filter and the oxygen concentration in exhaust gases then exceed their predetermined levels, by reducing the oxygen concentration in the exhaust gases to below the predetermined level.
This known art technique, however, does not approach the problem of dissolution loss of the particulate filter until the engine operating mode has shifted from a high load to idling condition and the temperature of the particulate filter increases, and does not intend to prevent the occurrence of temperature rise of the particulate filter on engine deceleration which means the period of transition from a high load to idling condition. Therefore, the above problem cannot be solved by this known art technique, nor has it still been solved.